A bicycle, also referred to as a bike or cycle, is a vehicle which provides for a human-powered, generally pedal-driven, means to propel the vehicle. Bicycles are generally single-track vehicles, having two wheels attached to a frame, one behind the other. Bicycles generally have a power driven rear wheel and a free spinning turnable front wheel.
Bicycles were introduced in the 19th century in Europe and number more than a billion worldwide. Bicycles are the principal source of transportation in many regions. They also provide a popular form of recreation, and have been adapted for use in military and police applications, in courier services, as children's toys, and in racing. Bicycles are now being adapted to include additional power sources and motors, to ease the physical burden on the user. Electric powered bicycles are often used as an alternative to vehicles such as cars. However, charging the power storage elements of these bicycles is often difficult. Additionally, theft and damage are a concern when parking a bicycle in a public place. It is not always possible to carry one's bicycle into buildings such as offices and schools.